The present invention concerns a detection process of formation fluid invasions in a well during drilling, as well as a device for the implementation of this process.
It finds application specifically in onshore of offshore oil drilling rigs.
Formation fluid invasions and specifically gaseous hydrocarbons in a borehole that occur during drilling or operations are phenomena that need to be detected as early as possible in order to facilitate their control and to minimize their consequences as uncontrolled eruptions can cause fires, pollution, the destabilization or the loss of the well.
The need to detect invasions early intensified with the development of borehole drilling in more and more difficult conditions, such as those encountered in drilling boreholes at high pressure and high temperatures, as well as in the drilling deep-sea boreholes in very great water depths.
Classically, a rig comprises a hollow cylindrical drill string arranged inside a casing, provided at its lower end with a drill bit and coupled at its upper part to a rotary drive device.
For offshore drilling, the rig includes in addition a pipe riser that joins the upper part of the surface conductor to surface equipment. During the drilling the fresh mud stored in tanks is injected inside the drill string by a mud pump.
After passage through the drill bit the mud injected inside the drill string is charged with rock cuttings formed by the action of the drill bit on the formation, and rises in the annular space defined on the one hand by the drill string and the casing extended by the pipe riser and on the other hand the borehole wall during the course of drilling, it then returns in storage pits after the elimination of cuttings.
A recognized method of detection of fluid invasions in the borehole during drilling consists in measuring the difference between the flow of fresh mud injected and the flow of charged mud that rises, to compare this difference then to a predetermined threshold. The exceeding of this threshold means a fluid invasion.
This flow difference is generally determined indirectly starting from the measurement of the mud level in storage pits, corrected for known additions of fluids to the mud, such as chemicals, and the mud volume variations in the borehole linked to tripping of equipment.
This determination is also affected by the loss of circulation in a part of the formation that can conceal or delay the difference variation between the fresh mud flow and the charged mud flow resulting from fluid invasions in the borehole, which difference is the basis of the detection.
The accuracy of such a determination, therefore, is very mediocre and insufficient to detect fluid invasions in boreholes efficiently and early.
In the case of drilling in very great water depths this precision is even worse, because of movements of the floating marine platform that carries the drilling facilities and because of the cyclic fluctuations of the pipe riser volume due to the heaving of the platform. This poor precision is even more bothersome in this case, where the formation fracture pressure at the fragile point of the borehole, decreases the maximum admissible volume of a fluid invasion without the risk of losing the well.
Another detection method of a formation fluid invasion is described in the document U.S. Pat. No. 4,733,232 dated Mar. 22, 1982. It consists in modulating the drilling flow close to bottom hole and comparing pressure impulses that are reflected at the time in the drill string and in the annular space. The ratio of the reflected impulse is modified by an invasion of gas that change the density of the mud that rises in the annular space, this modification is detected by a computer and is used as an alarm signal.
Another method of detection of a formation fluid invasion is described in the document EP 621 397 dated Jun. 25, 1991. It consists in analyzing vibrations emitted in the mud at the inlet of the drill string and vibrations sensed in the upper part of the annular space. These latter vibrations result from the propagation inside the drill string and in the annular space, of the vibrations emitted, and are affected by fluctuations of mud characteristics that change with formation fluid invasions in the borehole. The comparison by means of a vibration analyzer of the vibrations emitted and those sensed permit the detection of formation fluid invasions.
These last two methods are sensitive to cuttings entrained by the mud rising in the annular space and can in certain cases become inoperative due to the significance of reflection of the vibrations from the rock formation particles that constitute the cuttings.
In addition, their implementation requires the use of costly equipment.
The present invention has precisely as its object the remedying of these inconveniences and specifically to provide a process and a device for early detection of a formation fluid invasion in a borehole during drilling.
This process and this device may be used for the detection of fluid invasions in the form of liquid or gaseous hydrocarbons, or water, during the course of drillings both onshore and offshore, this detection permits taking counter measures to stop the development of these invasions or to minimize their effects.
For this end, the present invention proposes a detection process of a formation fluid invasion in a borehole during drilling, said drilling consisting of executing a borehole in a formation, by means of a rig including a hollow cylindrical drill string, arranged inside a casing, and into which string is injected fresh mud, said drill string, said casing and the borehole wall in the course of execution define an annular space through which the charged mud rises, which process is characterized in that it consists of:
measuring continuously the thermal flux circulating between the fresh mud and the charged mud through the wall of the drill string, at a given depth,
calculating continuously starting from said thermal flux, the value of a representative characteristic of a thermal balance as established in the absence of a formation fluid invasion, between the fresh mud circulating inside the drill string and the charged mud rising in the annular space, and
detecting fluctuations greater than a threshold, of the value of said characteristic, said fluctuations translate into a disruption of said thermal balance resulting from invasions of formation fluid in the borehole.
According to another characteristic of the invention the representative characteristic of the thermal balance is the speed of fluctuation of the thermal flux circulating between the fresh mud and the charged mud through the wall of the drill string, at a given depth.
According to another characteristic of the invention the representative characteristic of the thermal balance is the difference between the instantaneous value and a moving time window average value of the thermal flux circulating between the fresh mud and the charged mud through the wall of the drill string, at a given depth.
The drilling rig being operated from a command control system, according to another characteristic, the present invention proposes a process that consists in addition, in case of detecting a fluctuation of the characteristic, greater than a threshold, of transmitting a signal indicative of said detection to said system so that it generates an alarm.
The present invention also has as its object a detection device of a formation fluid invasion in a borehole during drilling, said drilling consisting of executing a hole in a formation by means of a drill rig comprising a hollow cylindrical drill string, arranged inside a casing and into which string is injected the fresh mud, said drill string, said casing and the borehole wall of the hole in progress define an annular space through which the charged mud rises, which device is characterized in that it includes;
at least one measurement sensor of the thermal flux circulating between the fresh mud and the charged mud through the drill string wall, at a given depth, sensor that provides a measurement signal of the said thermal flux on an output,
the calculation means connected to the output of said sensor, to calculate starting from the thermal flux measured, the value of a representative characteristic of thermal balance as established in the absence of a formation fluid invasion, between the fresh mud circulating inside the drill string and the charged mud rising in the annular space, and
processing means, connected to an output of the calculation means, to:
detect fluctuations in the value of the said characteristic, greater than a threshold, said fluctuations translating into a disruption of said thermal balance resulting from a formation fluid invasion in the borehole, and
deliver a signal indicative of a fluid invasion on an output.
According to another characteristic of the invention device, the representative characteristic of the thermal balance is the speed of variation of the thermal flux circulating between the fresh mud and the charged mud through the drill string wall, at a given depth.
According to another characteristic of the invention device the representative characteristic of the thermal balance is the difference between the instantaneous value and a moving time window average value of the thermal flux circulating between the fresh mud and the charged mud through the drill string wall, at a given depth.
According to another characteristic of the device of the invention, the rig operated from a command control system, said device includes the transmission means connected to the outlet of processing means, to transmit the signal indicative of a fluid invasion to said system, so that said system generates an alarm.
The present invention also has for its object a second process of detection of a formation fluid invasion in a sub-sea borehole during drilling, said drilling being realized by means of a drill rig including a cylindrical hollow drill string, arranged inside a casing extended by a riser pipe stretching in the water between the sea floor and the sea surface, some fresh mud being injected into said drill string that forms with said casing and said pipe riser an annular space through which the charged mud rises, which process is characterized in that it consists of:
measuring continuously the thermal flux circulating between the charged mud and the sea water through the wall of the riser pipe, at a given depth,
calculating continuously starting from the measured thermal flux, the value of a representative characteristic of a thermal balance as established in the absence of a formation fluid invasion, between the sea water and the charged mud rising in the annular space, and
detecting variations greater than a threshold, of the value of the said characteristic, said variations translating into a disruption of the said thermal balance resulting from invasions of formation fluid in the borehole.
According to another characteristic of the second process of the invention, the characteristic representative of the thermal balance is the speed of fluctuation of the thermal flux circulating between the charged mud and the sea water through the wall of the pipe riser, at a given depth.
According to another characteristic of the second process of the invention, the representative characteristic of the thermal balance is the difference between the instantaneous value and a moving time window average value of the thermal flux circulating between the charged mud and the sea water through the wall of the pipe riser, at a given depth.
The invention also has as its object a second detection device of a formation fluid invasion in a sub-sea borehole during drilling, said drilling being realized by means of a drilling rig including a hollow cylindrical drill string, arranged inside a casing extended by a pipe riser stretching in the water between the sea bottom and the sea surface, of the fresh mud being injected in said drill string that forms with said casing and said pipe riser an annular space through which the charged mud rises, which device is characterized in that it includes
at least one measurement sensor representative of a physical magnitude of the drilling operation, that provides a measurement signal of physical magnitude on an output,
calculation means, connected to the output of the sensor of a physical magnitude, to calculate from said physical magnitude, the value of a representative characteristic of a thermal balance as established, in the absence of a formation fluid invasion, between the sea water and the charged mud rising in the annular space, and
processing means, connected to an output of calculation means, to:
detect variations of the value of the said characteristic, greater than a threshold, said variations translate into a disruption of the said thermal balance resulting from a formation fluid invasion in the borehole, and
deliver a signal indicative of a fluid invasion on an output.
According to another characteristic of the second device of the invention, the representative characteristic of the thermal balance is the speed of fluctuation of the measured thermal flux.
According to another characteristic of the second device of the invention, the representative characteristic of the thermal balance is the difference between the instantaneous value and the moving average value of the measured thermal flux.
According to another characteristic of the second device of the invention the rig is operated from a command control system, it comprises the transmission means connected to the output of the processing means, to transmit to said system the signal indicative of a fluid invasion, so that said system generates an alarm.